It will be an epochal work in the field of nanotechnology if it is possible to control the structure and unique properties of metal oxides in nanometer scale. A technique of extending a one-dimensional nanostructure into a two-dimensional (2D) or three-dimensional (3D) regular through assembly is garnering a lot of attentions in the field of materials chemistry. It is because materials with different structures and crystal shapes are thought to have different electrical and optical properties in general. Accordingly, preparation of metal oxide with controlled structures and shapes is necessary.
Hematite (α-Fe2O3), an n-type semiconductor with a band gap of 2.1 eV, is the most stable, non-toxic, corrosion-resistant, magnetic iron oxide under ambient conditions. Due to its low cost, high corrosion-resistance and superior environment friendliness, this metal oxide has been actively studied for use in gas sensors, rechargeable lithium batteries, catalysts, biological and medical applications, magnetic recording, pollution control, optical instruments, or the like. Until now, there have been a lot of efforts to elaborately prepare iron oxides of various shapes including nanorod, nanotube, nanowire, nanobelt, hexagonal prism, spindle, snowflake, urchin, nanoring, flake, nanocrystal, nanocube and mesopore. However, there have been few researches the preparation of hematite superstructures in a simple and environment-friendly manner. Especially, a method of preparing novel hematite with desired structure and shape via a simple route without using a template has not been developed yet. For preparation of α-Fe2O3 nanoparticle, various techniques such as sol-gel method, microemulsion method, self-assembly, hydrothermal method, chemical precipitation and forced hydrolysis have been developed. Among them, the hydrothermal method has been widely used owing to easy control of nanoparticle size and shape, good uniformity, superior crystallinity of the product and relatively low reaction temperature.
However, it is not easy to grow a nanostructure of desired shape and size with orientation in general and use of a solid template is necessary. For control of oriented growth, various synthesis methods using templates such as porous alumina, polymer latex, silica and carbon, supermolecules, surfactant, organogel, etc. have been developed. However, use of a template is disadvantageous in that production cost and time are increased in addition to the inclusion of impurities. It is not easy to completely remove the template and the template often causes environmental problems and has a negative effect on the structure of the final product. Accordingly, development of a method capable of synthesizing novel iron oxide nanostructures via a fast and environment-friendly route without using a template is important.
In addition, a recent finding demonstrated that iron oxide nanoparticle can catalyze oxidations, similarly to natural peroxidase, giving rise to new possibilities in the field of enzyme mimics or artificial enzymes. Researches are actively carried out thereabout since the drawbacks of natural enzymes such as denaturation by proteases, requirement of special storage conditions and high cost can be resolved.
Shape of iron oxide nanoparticles is recognized as an important parameter influencing nanoparticle properties. Particularly, one-dimensional (1D) structures such as nanorod and nanowire possess unique properties that are different from their bulk materials and even the zero-dimensional counterparts (spherical nanocrystals). This is also seen in the recently developed peroxidase-mimicking nanoparticles where the peroxidase mimic activity of rod-shaped Fe3O4 dominates over the spheres which are much smaller in size. Accordingly, preparation of various iron oxide nanoparticles differing in physical parameters such as shape, size, surface area and dimensionality and studies on their effect on the enzyme mimic activity of the iron oxide nanoparticles are necessary.